1. Field of the Invention
The present invention relates to an antenna duplexer utilized for mobile communication or the like.
2. Description of the Prior Art
In mobile communication such as a radio telephone, a potable telephone and an automobile telephone have been put to practical use, consumer demand is for mobile communication apparatuses be which lightweight and small in size. In such apparatuses, transmission and receiving must be made by a common antenna, and a circuit for wave transmission and a circuit for wave receiving must be separated in a high frequency manner. Examples of an apparatus used for such purposes include an antenna switch and a circulator. When the frequencies of a transmitted wave and a received wave differ from each other, an antenna duplexer is generally used.
FIG. 6 is a perspective view showing a conventional antenna duplexer. This conventional antenna duplexer comprises a filter for wave receiving 50, a filter for wave transmission 60 and a matching circuit 70. The filter for wave receiving 50 has properties of passing a received wave and preventing a transmitted wave. In the filter for wave receiving 50, three coaxial resonators 51, 52 and 53 are connected to each other through a dielectric substrate 58, to constitute a polarized band-pass filter. The filter for wave receiving 50 comprises interstage coupling chip capacitors 55 and 56 and input-output coupling chip capacitors 54 and 57 which are connected to electrodes for coupling the coaxial resonators 51, 52 and 53. The dielectric substrate 58 on which the coaxial resonators 51, 52, and 53 are mounted is disposed on a dielectric substrate carrier 40 having a ground electrode formed on its reverse surface.
On the other hand, the filter for wave transmission 60 has properties for passing a transmitted wave and preventing a received wave. In the filter for wave receiving 60, three coaxial resonators 61, 62 and 63 are connected to each other through a dielectric substrate 68, to constitute a band-stop filter. The filter for wave transmission 60 comprises pattern inductors 64, 65, 66 and 67 formed on a dielectric substrate 68 connected to a capacitance forming electrode. The dielectric substrate 68 on which the coaxial resonators 61, 62 and 63 are mounted is disposed on the dielectric substrate carrier 40 having the ground electrode formed on its reverse surface.
The properties of each of the filters 50 and 60 are mostly determined by its input impedance. The filter is so constructed that the input impedance takes a value close to the standard characteristic impedance in a pass band, while taking a value deviating from the standard characteristic impedance, i.e., zero, infinity, or a pure imaginary number in a preventing band.
Mismatching of impedances is a problem when the filter for wave receiving 50 and the filter for wave transmission 60 are coupled in parallel. Specifically, an input impedance relative to a transmitted wave of the filter for wave receiving 50 or an input impedance relative to a received wave of the filter for wave transmission 60 take an finite value, so that the impedances are mismatched between both the filters 50 and 60 and an antenna shared terminal 1. In order to prevent this, the above described matching circuit 70 is provided between both the filters 50 and 60 and the antenna shared terminal 1. This matching circuit 70 is constituted by two strip lines 71 and 72 provided on the dielectric substrate.40. The strip line 71 is used as a matching circuit on the receiving side, and the strip line 72 is used as a matching circuit on the transmission side. Respective ends of the strip lines 71 and 72 are connected to the antenna shared terminal 1, the other end of the strip line 71 is connected to one end of the filter for wave receiving 50, and the other end of the strip line 72 is connected to one end of the filter for wave transmission 60. The respective ends of both the filters 50 and 60 are connected to the antenna shared terminal 1 through the strip lines 71 and 72, the other end 3 of the filter for wave receiving 50 is connected to a receiver circuit, and the other end 2 of the filter for wave transmission 60 is connected to a transmitter circuit.
In the conventional example shown in FIG. 6, the matching circuit 70 is realized by the strip lines. (transmission lines) having the standard characteristic impedance, for example, a characteristic impedance of 50 .OMEGA.. The phase of the wave is shifted in the matching circuit 70 to bring the input impedance of the filter in the preventing band near infinity, thereby to respectively match the input impedances of the filter 50 and the receiver circuit and the input impedances of the filter 60 and the transmitter circuit. An antenna duplexer thus constructed has a function of introducing the received wave from an antenna to the receiver circuit and introducing the transmitted wave from the transmitter circuit to the antenna.
FIG. 7 shows the characteristics of the antenna duplexer shown in FIG. 6. In FIG. 7, terminal numbers of an S parameter coincide with the terminal numbers shown in FIG. 6 (1: an antenna shared terminal, 2: a terminal for a transmitter circuit, 3: a terminal for a receiver circuit). The curves in FIG. 7 show variation of the parameter S with respect to frequency for three different reflection losses in decibels.